As a light beam controlling member suitable for reduced thickness and reduced weight, a light beam controlling member (so-called Fresnel lens) has been known since the past in which an incident area for light is formed having a serrated cross-sectional shape (referred to, hereinafter, as a Fresnel shape) divided into a plurality of divided areas having a concentric circular ring shape (orbicular). This type of light beam controlling member has been used, for example, in applications where thinness is particularly advantageous, and in applications where effects caused by generation of unnecessary light can be ignored (such as in magnifying glasses and lighting systems) (refer to, for example, Patent Literature 1).
When this type of light beam controlling member is assembled into a product for illumination purposes, a light source, such as a light emitting diode (LED), is positioned on the incident area side that is formed into the Fresnel shape, such that a center axis of the light emitted from the light source is coaxial with an optical axis of the light beam controlling member, and fixed thereto.
The Fresnel shape in this type of light beam controlling member may be a type including only a refractive surface that refracts the light emitted from the light source, or a type including a reflective surface in addition to the refractive surface. The latter type is more advantageous than the former in terms of efficiently capturing the light emitted from the light source (such as the LED) with a large spread angle.
Here, FIG. 11 is a diagram of an example of a conventional design for a light beam controlling member 1 including a reflective surface of this type.
As shown in FIG. 11, the light beam controlling member 1 is configured by a disk-shaped light beam controlling section 2 including an optical axis OA that contributes to light beam control, and a cylindrical edge section 3 surrounding the light beam controlling section 2. The light beam controlling member 1 can be integrally formed using a mold, such as by an injection molding method using a transparent resin material, such as poly(methyl methacrylate) (PMMA), polycarbonate (PC), cyclo-olefin resin (COP), epoxy resin (EP), or silicon resin.
As shown in FIG. 11, the light beam controlling section 2 has two light beam controlling surfaces 4 and 5 that are an incident area 4 and an exit area 5. The incident area 4 and the exit area 5 face each other in the optical axis OA direction. In addition, the light beam controlling section 2 of the light beam controlling member 1 shown in the cross-sectional view in FIG. 11 has a circular shape from a planar view.
Here, as shown in FIG. 11, light L that has been emitted from a light source 6, such as an LED, enters the incident area 4. The light source 6 is disposed in a position facing the incident area 4 on the optical axis OA However, the light source 6 is configured to emit, towards the light beam controlling member 1 side, light L having a predetermined spread angle in relation to the optical axis OA direction. In addition, the center axis of the light L emitted from the light source 6 is aligned with the optical axis OA of the light beam controlling member 1 in the design. In FIG. 11, only the optical path of the light L emitted from a single light emission point on the optical axis OA in the light source 6 is shown. However, in actuality, surface emission is performed in the overall light source 6.
On the other hand, the light L from the light source 6 that has entered the incident area 4 travels the interior of the light beam controlling section 2 and enters the exit area 5 from within the light beam controlling section 2 (internal incidence). The internally incident light L is emitted from the exit area 5 towards the surface to be irradiated.
The incident area 4 will be described in further detail. As shown in FIG. 11, the incident area 4 includes a circular center section 8 of which the center is the optical axis OA, and a plurality of projecting sections 11 surrounding the center section 8.
As shown in FIG. 11, the plurality of projecting sections 11 are adjacent to each other in a radial direction (horizontal direction in FIG. 11).
The projecting sections 11 form concentric circular ring shapes of which the center is the optical axis OA, when viewed from the optical axis OA direction. In addition, as shown in FIG. 11, the projecting sections 11 form a serrated cross-sectional shape in the optical axis OA direction (vertical cross-sectional shape), configuring the Fresnel shape as a whole.
Furthermore, as shown in an enlarged cross-sectional view in FIG. 12, each projecting section 11 has a first surface 14 and a second surface 15. The second surface 15 is formed in a position on the outer side in the radial direction with reference to the optical axis OA (inner end in the radial direction), in relation to the first surface 14. The first surface 14 is formed into a cylindrical surface of which the center is the optical axis OA. On the other hand, the second surface 15 is formed into an angled surface (tapered surface) of which the center axis is the optical axis OA, having a tilt angle that is a predetermined acute angle in relation to the optical axis OA, such as to tilt to the optical axis OA side towards the light source 6 side (lower side in FIG. 12). The first surface 14 and the second surface 15 are joined together at the respective tip end sections (lower end sections in FIG. 12).
Here, the light L emitted from the light source 6 enters the first surface 14 and is then refracted by the first surface 14 towards the second surface 15 side.
On the other hand, the light L from the light source 6 that has been refracted by the first surface 14 enters the second surface 15 from within the projecting section 11 at an angle of incidence that is a critical angle or more. The incident light L is totally reflected by the second surface 15 towards the exit area 5 side, or in other words, the surface to be irradiated.
The second surface 15 is formed having a rotationally symmetrical shape of which the symmetry axis is the optical axis OA. Therefore, a conical light is emitted in the optical axis OA direction from the overall second surface 15.
The light that has been totally reflected by the second surface 15 as described above reaches the exit area 5 and is emitted from the exit area 5 towards the surface to be irradiated.
In the light beam controlling member 1 configured as described above, the light emitted from the light source 6 can be efficiently captured by the first surface 14 of the sharp projecting section 11 that has been formed having sufficient height in the optical axis OA direction (such as 0.1 mm). Light beam control can be performed such that most of the captured light is totally reflected by the second surface 15 onto an optical path towards the surface to be irradiated. As a result, desired light distribution characteristics can be achieved.    Patent Literature 1: Japanese Patent Publication No. 2007-134316